A few months back, a question was asked on OpenBSD's official misc@ mailing list about the history of /usr. Was this originally a shortened form of "user", or what's this about "system resources"? Perhaps not a barn burner (translate: exciting ) of a subject, but some might like the perspective offered:

In particular, in our own version of the system, there is a directory "/usr" which contains all user's directories, and which is stored on a relatively large, but slow moving head disk, while the othe files are on the fast but small fixed-head disk.

So system stuff was traditionally kept in /bin and possibly /sbin, while "shared" user stuff may have been in /usr/bin instead.

I have a few older commercial Unix flavours for x86, they also typically put user stuff in /usr.

Having pages like hier(7) are really helpful, all systems should document their preferred directory structure.